The present invention refers to a method of detecting workpieces in an electrostatic coating system, as well as an electrostatic coating system in which this method can be used.
Conventional automatic coating systems are controlled by storage program controls (SPC). For this purpose a central computer unit is provided from which a plurality of electric and pneumatic lines extend to sensors and actuators of the system.
The current states of the sensors (e.g. supply clock of the workpiece, level of the reservoir etc.) are cyclically detected by the central unit, the necessary reactions of the associated actuators are calculated and appropriate control commands are supplied to the actuators.
In the conventional electrostatic powder coating system, a workpiece passes in the horizontal direction through a coating compartment with vertical slots being provided in the side walls of the coating compartment. Through these slots electrostatic coating guns discharge coating powder into the coating compartment. While the workpiece is passed through the coating compartment, the successively arranged plurality of coating guns reciprocate in the vertical direction, wherein the workpiece is coated by a plurality of adjoining or partially overlapping sine-shaped powder clouds due to the workpiece movement in the horizontal and the coating gun movement in the vertical direction.
In order to achieve a regular and sufficient coating thickness on the workpiece and in order to optimize the efficiency during powder application, the vertical stroke of movement of the coating devices and the powder discharge are controlled. Additionally, the spacing between the coating device and the workpiece is adjusted in the spray direction in order to prevent the powder from being blown off again in case the spacing is too low or to avoid the creation of electrostatic craters, or in the reverse case that the efficiency of the powder coating is aggravated and e.g. the penetration ability into the cavities decreases.
For this purpose, known powder coating system have a workpiece detection and identification means as well as a timer means.
The workpiece detection is used for instance in coating apparatus for realizing a gap control. For this purpose the supply and therefore the application of powder lacquer or wet lacquer is interrupted in workpiece gaps. Thereby the consumption of coating materials, the amount of waste of wet lacquers and the recirculated portion of powder lacquers is reduced. The basic design of a gap control of the prior art is shown in FIG. 9.
A workpiece feed device 902 transports workpieces 904 through a coating compartment 901 in the direction indicated. To realize the gap control a device for detecting a workpiece 904 is required. This device consists of a light barrier 906 or 906', 908, wherein light is transmitted by the light emitter 906 or 906' and received by the receiver 908, and wherein respective signals are passed on to a control 912.
The light barrier is preferably located outside the coating compartment 901. For the time-variant control of the material supply, the control 912 requires an additional signal which is proportional to the speed of the feed device 902. This signal directly comes from a feed system control 915 or it is determined by a special means 913 for measuring the feed speed. This speed measuring device generates a signal proportional to the feed speed and passes it on to the control 912. From the speed information and the signal of the light barrier 806, 908 the control 912 determines the time required by the workpiece 904 to reach the spray device 905.
The light barrier 906, 908 also "informs" the control 912 about the end of a workpiece. This is a device which detects the entry of a workpiece 904 into the compartment 901, which detects the length of the workpiece and which is capable of turning on and off the material supply 910, 911 in a time-delayed manner in accordance with the speed of the feed device 902.
To control the powder devices, the prior art further provides a position control means and a motion control means, which also require the information about the start, the end and the speed of the workpiece. The position control controls the distance of the coating devices to the workpieces. The motion control means controls the vertical movement of the coating devices.
The coating system requires a great calculation effort in order to switch the plurality of coating system on and off in a synchronized manner, and to reciprocate them up and down and back and forth so that an optimum coating result is achieved.
Furthermore, a relatively complex and expensive detection device is required for the workpiece detection and identification.
The object of the invention is to provide a method of detecting a workpiece in an electrostatic coating system, and a new coating system in which the amount of hardware and software for the workpiece detection and identification can be reduced and the control can be simplified.
This object is achieved by a method having the features described in the claims.